Communication devices such as wireless terminals are also known as e.g. User Equipments (UE), mobile terminals and/or mobile stations. Wireless terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two wireless terminals, between a wireless terminal and a regular telephone and/or between a wireless terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Wireless terminals may further be referred to as mobile telephones, cellular telephones, laptops, tablet computers or surf plates with wireless capability, just to mention some further examples. The wireless terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node or a base station. A cell is the geographical area where radio coverage is provided by the base station or access node.
A wireless communication network may include a number of cells that can support communications for a number of user equipments. A user equipment may communicate with a serving cell and may need to make measurements of other cells for various purposes. For example, for a user equipment to be able to monitor and eventually perform a handover to a neighbour cell, a timing of the neighbour cell needs to be known or measured. A timing of a cell is here referred to as the arrival time of a signal sent from the base station in the cell relative to a clock in the user equipment. The timing of a cell may hange due to a number of reasons, e.g. drift due to that the user equipment clock differs from the one used in the base station and drift due to a relative movement of the user equipment with respect to the base station. In order to adjust the timing of a cell, a time difference between the actual arrival time and expected arrival time of a signal sent from the base station in the cell is measured. The time difference hereafter is referred to as a timing offset of a cell.
In poor radio conditions producing a reliable measurement of the timing for all cells may not always be possible. Some cells may be heavily interfered by other cells or other disturbances which makes measurements of the timing offset unreliable. An erroneous correction of the timing of a cell may lead to that the user equipment loses the timing of the cell completely and thus have to reacquire the timing, using a cell search like method. This in turn will prevent it from measuring signal power etc. for that particular cell until the timing has been reacquired. In poor radio conditions this time may be considerable. The 3rd Generation Partnership Project (3GPP) requirements for cell detection are typically 20 times longer than the reporting interval.
To prevent this, user equipments typically suppress timing corrections in case a timing offset estimate is deemed to be too unreliable. However, in case of large timing drifts, either due to discontinuous reception and/or high relative velocities between the user equipment and the base station, suppressing timing corrections for an extended period of time will also cause the user equipment to lose the timing of the cell.